Converter circuits are known to include a converter unit with a multiplicity of controllable power semiconductor switches which are connected in a known manner for the purpose of switching at least two switching voltage levels. Such a converter circuit can be connected to an electrical AC voltage network which is in the form of a single-phase or three-phase network, for example. Such converter circuits are often used in industrial installations, where the AC voltage side of the converter unit of the converter circuit is coupled to the network via a transformer. Other fields of application and possible uses, for example wind power installations, are also naturally conceivable. FIG. 1 shows, by way of example, such a known converter circuit.
When connecting the transformer to an AC voltage network, the transformer may become saturated. Such saturation may also occur when the voltage returns following network faults. The extent of the saturation depends on the voltage/time integral during the network fault, that is to say is predominantly given by the AC voltage network. Following such a fault or when connecting the transformer to the AC voltage network, the magnetic flux of the transformer can have a significant DC component which then results in saturation. However, the saturation of the transformer is highly undesirable. FIG. 2 illustrates, by way of example, such a typical temporal profile of the magnetic flux of a winding set of the transformer of the converter circuit according to FIG. 1.
U.S. Pat. No. 6,577,111 B1 specifies a method of the generic type for operating a converter circuit, which method is able to compensate for undesirable saturation of a transformer of the converter circuit but does not return the magnetic flux to below the magnitude of a nominal value of the magnetic flux of the transformer.